Conventional centrifugal seals employ the principle of centrifugal effect at high speeds to retain a fluid for a sealing purpose. Such conventional centrifugal seals are typically secured to a rotating shaft member and use a lubricating liquid, such as oil, to effect the seal when the shaft member has reached a high speed. Such conventional centrifugal seals of this type function satisfactorily in the high-speed mode, but, at lower shaft speeds, the seal fluid is not retained in place, while, in the static mode, the seal does not support any pressure at all. Other difficulties associated with conventional centrifugal seals include problems associated with fluid leakage, limited life due to elastomer-seal wear and high torque and high static friction, which result from the use of elastomeric seals. Thus, a completely hermetic, high-speed, centrifugal shaft seal, which has sealing properties both at the static and low- and high-speed modes and with increased seal life and lower torque and static friction, would be most desirable.
Rotary-shaft seals employing ferrofluid, typically for vacuum applications, are known, and such seals are designed to withstand a pressure differential of up to 1 atmosphere under static, as well as dynamic, conditions. A multiple-stage, rotary-shaft ferrofluid seal is shown, for example, in U.S. Pat. No. 3,620,584, issued Nov. 16, 1971, hereby incorporated by reference. The seals employ a magnetic circuit consisting of a permanent magnet and pole pieces to provide the necessary magnetic flux to entrain discrete, magnetic-ferrofluid liquid O-rings in each annular stage of the seal, with the rotary shaft completing the magnetic circuit. Each ferrofluid O-ring is a total hermetic barrier to the passage of gas and typically may have a differential pressure capacity of 20 to 35 kPa (3 to 5 psi). The total pressure capacity of the multiple-stage ferrofluid seal is the sum of the pressure capacities of the individual stages in the seal.